a) Field of the Invention
The present invention relates to an optical pickup apparatus used in an optical disk device, in particular, it relates to an optical pickup apparatus which can be commonly used for recording media having protective layers with a different thickness.
b) Description of the Related Art
Conventional compact discs ("CDs" hereafter) have been widely used as optical discs on which information signals are optically recorded. Recently, with progress in technology to densify optical discs, digital video discs ("DVD" hereafter), in which animated images with a length of several hours can be written in and read from an optical disc with a diameter same as a CD, has come into actual use. Currently, various kinds of optical pickup apparatuses are suggested for reading both CDs and DVDs. Although CDs and DVDs share basic principles, they differ in the thickness such that CDs have a thickness of 1.2 mm while DVDs have a thickness of 0.6 mm, which is a half of the thickness of CDs. Therefore, in order to realize an optical pickup apparatus for reading both CDs and DVDs, it is required to be structured such that spherical aberrations due to differences in the disc thickness are canceled.
Examples to realize an optical pickup apparatus for reading both CDs and DVDs are an objective lens switching method and a compensation device method. Both of these methods utilize a common light source. In the objective lens switching method, two objective lenses facing optical discs are used for CDs and DVDs, which are switched according to which type of disc is read; in the compensation device method, spherical aberrations due to the differences in the disc thickness are canceled by a compensation device.
Both CDs and DVDs are optical discs which share basic principles. However, in an optical pickup device for CDs, a semiconductor laser with a wavelength of 780 nm as a light source is used, while a semiconductor laser with a short wavelength of 650 to 630 nm is used as a light source in an optical pickup device for DVDs. Hence, in order to enable reading of both CDs and DVDs using a common light source as described above, a semiconductor laser with a short wavelength of 650 to 630 nm shall be used as a light source. Also, there is no negative effect, such as damaging a reflective film of CDs, caused by reading CDs with such a short wavelength laser. As a result, if a semiconductor laser with a short wavelength of 650 to 630 nm is applied as a light source, the semiconductor laser can be commonly used for CDs and DVDs by switching objective lenses or by using a compensation device.
Nonetheless, CDs have been developed in various modes; for example, CD-Rs are capable of addition and writing. The reflective film of the CD-R is designed such that maximum performance can be obtained by using a laser for CDs with a wavelength of 780 nm, therefore, the reflective film is said to be highly dependent of wavelength. As a result, in the case of using a semiconductor laser with a short wavelength of 650 to 630 nm in order to be applicable for both CDs and DVDs, as described above, the reflective film of the CD-R cannot reflect the short wavelength laser beam, therefore, information signals recorded on the CD-R cannot be reproduced. In addition, when such a short wavelength laser beam is irradiated on the reflective film of the CD-R, the reflective film is heated by absorbing the short wavelength laser beam such that it could be damaged.
In order to realize an optical pickup apparatus which can be commonly used for CDs and DVDs and which can read CD-Rs, in general, it is possible to use a plurality of kinds of light sources corresponding to kinds of recording media, as well as to connect detecting devices corresponding to each of the light sources for selecting a light source and a detecting device corresponding to the kind of used recording media. However, an increase in the number of parts causes increased costs in addition to inconvenience that the detecting device has to be switched corresponding to the kind of the used recording medium.